This invention relates generally to polymer materials and more specifically to a crosslinked polymer particularly suitable for use in an implanted biomedical application.
The need currently exists for a low-toxicity, relatively low temperature, heat shrinkable polymer material capable of being used long-term in vivo, for example, as a connecting sleeve in a vascular anastomosis system, such as is disclosed in U.S. Pat. No. 4,470,415 issued to J. J. Wozniak. In that patent, a heat shrinkable sleeve material is placed over abutting ends of the vascular members to be joined; the respective ends of which have been prepared to be anastomosed by everting them over ferrule members placed over the ends of the vascular members. When the sleeve material is subjected to heat, it contracts and engages the vascular members and maintains them in firm connection.
Clearly, for such implanted biomedical applications as this, it is essential that the sleeve material have minimal toxicity and otherwise be compatible with implantation for the intended period of use, as well as being shrinkable at a temperature which is not so high as to cause necrosis of the adjacent body tissue. It should be understood, of course, that the proposed heat shrinkable material of the present invention has numerous applications, in addition to the implanted biomedical application such as that just described.
The crosslinking of polymeric substances by subjecting them to irradiation has been studied and utilized for several years, e.g. as a means of altering various structural parameters of the material. For example, A. Charlesby, in British Pat. No. 732047 published June 15, 1955, disclosed the treatment of polymeric substances to increase their resistance to organic solvents, by irradiating them with high energy electrons or gamma rays to thereby produce intramolecular bonds or crosslinks. Charlesby specifically disclosed the treatment of polyethylene, polystyrene, polyvinyl chloride, nylon, neoprene, gutta percha, smoked rubber, polyvinyl acetate, rubber hydrochloride and polyvinyl alcohol.
Similarly, W. A. Patterson, in U.S. Pat. No. 3,429,794, teaches a solvent shrinkable polymeric material produced by exposing the polymer to radiation from such sources as high energy electrons or the gamma rays from Cobalt 60 and then orienting it by stretching.
W. G. Baird, Jr., in U.S. Pat. No. 2,943,370, also teaches the use of high energy electrons and Cobalt 60 gamma rays to produce a heat shrinkable plastic made from polyethylene and useful for the production of document copies.
Other crosslinked polymers produced by irradiating the substance from a high energy source such as high voltage electrons or gamma rays, in order to enhance their physical or mechanical properties such as infusibility or solubility, are taught in the Charlesby et al U.S. Pat. No. 3,372,100; whereas, the manufacturer of a heat-recoverable crosslinked polymer of vinyl chloride and a polyunsaturated monomer are taught by Pinner in U.S. Pat. No. 3,359,193.
An implanted biomedical use of a heat shrinkable polymeric tube is taught by Bokros in U.S. Pat. No. 4,169,477, to join a vascular graft to the tubular portion of a prosthetic device; e.g. to allow accessing of a patient's blood system. In particular, this patent teaches the use of a copolymer of tetrachloroethylene and hexachloropropylene, trade-name TEFLON-FEP, which is described as being heat shrinkable at a temperature of approximately 300.degree. F.
The above-described prior art thus teaches several heat shrinkable materials, but which for one reason or another are unsuitable for a significant number of biomedical and other applications. In particular, for implanted biomedical use, the heat shrinkable material must be biocompatible and, to enable a sleeve of the material to be applied in vivo, for example, as part of a vascular anastomosis procedure, it must be heat shrinkable at a temperature non-injurious to surrounding body tissue.
There are general criteria useful to assess the biocompatibility of the proposed heat shrinkable polymer for implanted use. First, the material must have low toxicity as regards local tissue response (necrosis or inflammation), systemic reaction and allergies, and be non-carcinogenic. Secondly, the polymer material must retain its structural form and perform its intended function over the anticipated life of the implant; specifically, the material must not dissolve or deteriorate when subjected to body fluids and enzymes. Finally, the temperature needed to initiate shrinkage and the duration of applied heat must not cause tissue necrosis. Conversely, the shrink temperature should be above normal body temperature and should also be selected to permit use in elevated temperature, e.g. tropical environments.